I did two trips to Philmont. 10,000 feet is the highest we hiked there. Did a trip in the Rockies once a few years later with a couple of buddies. After 3 days in the car we hit the trail as soon as we arrived. I had to stop about 2 miles from our destination as my legs cramped up bad. Went to sleep and didn’t wake until the next morning. Felt 100% then.
The San Jacinto trip usually started in Idyllwild at the foot of "Devil's Slide". We hiked to the "saddle", then headed the long way around to Round Valley to camp for the night. In the morning, we walked to Wellman's Divide and dropped our packs. One of the adults watched the packs while the rest of us raced unfettered to the summit. The San Gorgonio summit was always a fully packed experience. There wasn't a convenient spot to drop the packs.
The leg cramps seemed to be most apparent in the first 3 miles of walking. After that, they tend to relax as long as you maintain a decent pace.